Running used to be a simple endeavor. All you needed was a shirt, shorts and shoes. Now, it’s far more complicated. We are a nation of accessorizers, even in our exercise. We carry water bottles, music players, phones and activity trackers. And although we use them to make our runs easier, here’s the bad news: They could be slowing us down.

To understand why an MP3 player or a water bottle might hinder your progress, a brief lesson in biomechanics is in order. Your upper body plays a critical role when you run. (That’s why the bad guys never get far when they escape from police custody in handcuffs.) Both the arms and the torso come into play, helping the legs lift the body and working together to create a smooth stride.

Good running form starts in the hands. They should be relaxed and comfortable. If you’re holding something, you’ll create tension and imbalance in your upper body. No matter the object — a water bottle, an iPod, a set of keys — holding something alters your form and makes you exert more energy. And the more effort you expend, the faster you’ll tire.

To see how this happens, pretend to grip a bottle and move your arms as you would while running. Even without the bottle, your forearm muscles contract. Or try running with your fists clenched. That tension in your hands creeps to your forearms, then your upper arms. This makes shoulder rotation more difficult, which inhibits your leg drive. To become more relaxed, hold a saltine cracker between your thumb and forefinger, and try not to break it while running. It’s easy to see how even an empty water bottle or an iPod could have a detrimental effect on your gait.

On a physiological level, when you run, your blood gets redistributed to the areas of your body that need it. As your hand and forearm muscles contract, blood flow to those places increases. But as you power up that hill, your blood has better places to be — like your legs. To the casual runner, this diverted blood flow means a less enjoyable run (or a more painful one).

The bigger problem, however, is that these objects make your form asymmetrical. Jonathan Cane, founder of City Coach Multisport in New York City and co-author of “The Complete Idiot’s Guide to Weight Training,” says he can always spot people holding something. “These people have what I call ‘iPod arms.’ One arm moves less than the other.” When one arm has less motion than the other, one stride will be shorter than the other, hence the asymmetry. “People always underestimate the role of arms in running,” says Cane, who has been training endurance athletes for two decades. “What happens above the waist impacts what happens below. A poor stride is often the result of minimal shoulder flexibility.”

Form imbalance is not only inefficient, it could lead to injury. When your arms are unbalanced — that is, when you hold something in one hand but not the other — everything about your form is unbalanced. You might end up putting more stress than usual on a muscle group. Or you might stress one side more than the other. This might not seem like a big deal, but multiply that one stride by the thousands you take over the course of a run, and it adds up. One side of your body might fatigue sooner than the other, and this fatigue and muscle imbalance could lead to injury in the long term.

A good, efficient running form, then, starts with proper distribution of weight across your body. “Mass distribution is one of the most important elements when it comes to biomechanics. The closer the object is to your torso, the less effect it will have on your form,” says James Smoliga, associate professor of physiology at High Point University in North Carolina. That’s the problem with your hands: They are far from your torso. The lower any mass is distributed down your arm, the more impact it will have. This is why an iPod strapped to your upper arm won’t affect form as much as an iPod in your hand.

Smoliga, a former collegiate runner who researches the effects of biomechanics and the neuromuscular system on endurance sports, says that the farther from your body you carry an object, the more force your muscles will need to generate to carry it. Translation: It will tire you out faster. A 3-pound weight strapped to your waist might slow you a little, but strap that weight to your shoes or your hands and you’re a sloth. Although the ideal amount of excess weight during exercise would be zero, Smoliga says that if you run with water, make sure it’s as close to the body as possible, like the hydration systems that you strap to your waist or the center of your back. Still, he cautions, any excess weight will alter your biomechanics, especially over a long run.

But here’s some good news: You most likely don’t even need water on your run. Although hydration is important, there’s a good chance you’re probably already hydrated before you start running. There are exceptions, of course, depending on the length and intensity of your run or training session. But if you stay hydrated throughout the day, you shouldn’t need that water bottle in the first place. The level at which we perceive thirst is still a safe one unless you’re competing. “The average healthy person can probably exercise for an hour without worrying about fluid replacement,” Smoliga says. “That slight feeling of thirst is not a major factor when you’re out on a training run, so long as you can readily rehydrate soon after.”

Still worried about dehydration? Here’s how to tell whether you need to carry water: Assuming you start off hydrated, weigh yourself before and after your run. A difference of more than 2 percent means carrying water is a good idea in the future — just not in your hands. You can also take steps to minimize dehydration by starting your run hydrated, staying in the shade as much as possible, going out during the cooler parts of the day and wearing breathable clothing. As long as the clothing is light, of course.

Ben Opipari is a former track coach and founder of Persuasive Matters, a legal writing consulting company. Contact him at ben@persuasivematters.com.

More in News

Thirty-six people — musicians, artists, students, lovers and friends — lost their lives on Dec. 2, 2016, in the fire that consumed the Oakland warehouse known as the “Ghost Ship.” Here are their stories.

A long-awaited plan to keep the Raiders in Oakland was announced late Friday by city and council officials. It includes a public investment of $350 million, pegged to the value of the Coliseum land and infrastructure improvements.